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                [Usaco2004 Nov]Til the Cows Come Home 带奶牛回家                </h1>
                <p>时间限制：1s&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;  空间限制：128MB</p>			</div>
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                                <h3>题目描述</h3><p><div><span style="font-size: medium">贝茜在谷仓外的农场上，她想回到谷仓，在第二天早晨农夫约翰叫她起来挤奶之前尽可能多地睡上一觉．由于需要睡个好觉，贝茜必须尽快回到谷仓．农夫约翰的农场上有N(2&le;N&le;1000)个路标，每一个路标都有唯一的编号（1到N）．路标1是谷仓，路标N是贝茜一整天呆在那里的果树园．农场的所有路标之间共有T(1&le;T&le;2000)条不同长度的供奶牛走的无向小路．贝茜对她识别方向的能力不是很自信，所以她每次总是从一条小路的头走到尾，再以这条路的尾作为下一条路的头开始走．&nbsp;&nbsp;现给出所有路标之间的小路，要求输出贝茜回到谷仓的最短路程（每组输入数据都保证有解）．</span></div></p><hr/><h3>输入格式</h3><p><div><span style="font-size: medium">第1行：2个整数T和N．</span></div>
<div><span style="font-size: medium">第2到T+1行：每行用空格隔开的三个整数描述一条小路．前两个整数是这条小路的尾和头，</span></div>
<div><span style="font-size: medium">第三个整数是这条小路的长度（不大于100）．</span></div></p><hr/><h3>输出格式</h3><p><div><span style="font-size: medium">一个整数，表示贝茜从路标N到路标1所经过的最短路程</span></div></p><hr/><h3>样例输入</h3><pre>5 5
1 2 20
2 3 30
3 4 20
4 5 20
1 5 100</pre><hr/><h3>样例输出</h3><pre>90
//共有5个路标，贝茜可以依次经过路标4，3，2，1到家</pre><hr/><h3>提示</h3><p>没有写明提示</p><hr/><h3>题目来源</h3><p>Gold</p>
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